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Removing Negative Feedback Loop in Super Reverb?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Pedro58, Apr 3, 2005.

  1. Pedro58

    Pedro58 Supporting Member

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    I did this before and liked the results. I loaned the amp to a studio and they put it back to increase clean headroom. I remember taking a wire off the speaker jack...
    Any one know how to do this? Advice? Dangers to the amp/transformers, tubes, etc.? Dangers to the idiot (Me!) fiddling around in there? Thanks!
     
  2. Wakarusa

    Wakarusa Member

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    I remember taking a wire off the speaker jack...
    Specifically, it is a wire going from the speaker jack to an 820 ohm resistor about 7/8 of the way down the circuit board. The other end of the 820 goes to a common junction with a 0.1uF cap and 100 ohm and 22K resistors.

    Any one know how to do this?
    Yes, remove the wire.

    Advice?
    Be careful

    Dangers to the amp/transformers, tubes, etc.?
    None.

    Dangers to the idiot (Me!) fiddling around in there?
    You can electrocute yourself if you don't take care to discharge filter caps. You can burn yourself with the soldering iron. You can drop the chassis on your toe.

    Thanks!
    You're welcome.
     
  3. Pedro58

    Pedro58 Supporting Member

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    Thanks for the specifics, Todd. And the humor. I know it's a greenhorn question. :D
     
  4. Wakarusa

    Wakarusa Member

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    Not at all :D

    If you don't want to go through the hassle of completely removing the wire (since it threads through a hole in the eyelet board) you can just unsolder it from the speaker jack and then insulate the bare end with a bit of heat shrink.
     
  5. The other guys did already give some excellent info but...

    please be carefull 'cause sometimes there still can be some high voltages stored in the power-supply-caps!
     
  6. Pedro58

    Pedro58 Supporting Member

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    I know about discharging the caps...
    And I think sealing the wire with heatshrink was what I did last time. Can I put a switch on it through the extra speaker jack? I've heard of that. It would be nice to be able to go back and forth, maybe. I'll try it and report my findings... when I find the time to do it!
     
  7. Wakarusa

    Wakarusa Member

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    A switch on it works fine.

    If you're feeling adventurous you can install a potentiometer with switch (and a couple of other tidbits) instead -- presence control on the pot, switch to remove NFB.
     
  8. Pedro58

    Pedro58 Supporting Member

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    Is the difference between NFB and no NFB so great that the sweep on a pot would be usable? What I mean is that the switch "on" and switch "off" is not a night and day difference. Therefore, the range bewteen them might not warrant the use of a pot. Unless, maybe, you change the value of that resistor and on and on... Those kinds of changes are beyond my abilities.

    A reputable amp tech told me that to put a pot there would be a waste of time, in his opinion. Granted, he told me that when I was trying ot get him to draw me a schematic for one! But his point, and I think it's a valid one, was that I was chasing a tone/flexibility mod that just wasn't that big a deal.
     
  9. Wakarusa

    Wakarusa Member

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    You need more than just the pot to make things interesting. The idea is to install a presence control (which can have a significant effect). To make it work well requires a bit of design savvy. I think that this may be something for you to refer to a competant tech if you want to pursue it.

    For your immediate purposes (with or without NFB) a single switch is sufficient.
     
  10. Pedro58

    Pedro58 Supporting Member

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    Thanks, TOdd. That's what I thought. Real tweaking is best left to people who know what they're doing... I only know enough to ruin the amp!
     
  11. Scottone

    Scottone Supporting Member

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    I have a feedback control on my London Power Studio amp and the effect is indeed subtle when only changing the settings slightly.

    I like the idea proposed by Wakarusa to add a presence control with a switch to remove the NFB. This would of course require some experimentation to figure out the appropriate pot and capacitor values.
     
  12. Wakarusa

    Wakarusa Member

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    The presence control bypass cap isn't so hard to figure. It is the relationship between the NFB resistor, presence pot, and "tail" resistors that requires a bit of thought. The amount of NFB has more to do with the ratios of all of this stuff than their absolute values, and the range (and therefore influence) of the presence control is (again) a ratio with the rest of the NFB circuit.
     
  13. Pedro58

    Pedro58 Supporting Member

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    Hey, Todd! I just realized that you're an amp guy and your last name is "Shock!" Funny! I'm sure you've heard it before, but I just had to be the next idiot to say something...
    Your last post was over my head, tech-wise. I can only surmise that a tail resistor is a man who eschews the pursuit of... Oh, never mind! Thus, for me, a simple switch is best. Good ideas, though!
     
  14. BluzMike

    BluzMike Guest

    Not long ago I traded in a '65 Deluxe Reverb. A dumb guy had replaced the bias pot next to the voltage selector (export model). So what to do with that stupid hole? Adding a master? To a 20W amp? Nope!
    So I went for the presence control/driver circuit of a blonde Bassman. Great results. The tone thickened up a bit, but without getting mushy. There is a bit more control on the top end. And some growl was added. Overall, the amp is sounding bigger and a bit more dgey, without leaving its heritage.
    Strongly recommended.
    Michael
     

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